It seems like the series has been wanting to play sets of ponies off of each other. Rarity and AJ are the hard-working ones that manage to get in trouble by getting too much work and half-truths, but they both of very different views on social etiquette. Twilight and Pinkie make the "Dexter's Lab" pair where you have the bookish genius confounded by a bright but almost alien intellect.

Then why Fluttershy and Dash? They don't seem similar enough for their differences to have much weight. I theorize that trying to answer this has lead to the path of least resistance for the community: Shipping.

They were both pushed hard in their youth. They both went to flight school. They both failed. They reacted very differently.